Real world P-51 supercharger cut in and throttle handling. Reduce throttle anticipating cut in.

[-0303-]

Been wondering about this behavior. Like, should you keep advancing throttle to keep power up until supercharger kicks in while climbing? Ok, the MP jump on supercharger kick in, that's how it worked.

 

http://www.aviation-history.com/engines/merlin.htm

Quote

    There was no doubt when the supercharger cut into the high-blower position on the P-51 Mustang. The aircraft shuddered violently and pilots had to learn to anticipate the cut-in and reduce throttle. When descending, the change to low-blower took place at about 14,500 feet, and the only indication of the event was a drop in manifold pressure.

 

Edited May 8, 2021 by -0303-

[reece146]

Been wondering about this.

 

I've been flying the Spit mostly lately and have taken to dropping engine power just before the high speed blower kicks in  - "just 'cuz" it seemed like the mechanically sympathetic thing to do.

 

 

[grafspee]

I always increase rpm to max before cut in

[sublime]

As it is now when the supercharger kicks in it makes the engine sound a lot louder. Which is ok but it can make the audio sound a bit off, especially if you go down then up a few times. The non s/c engine sounds are almost too quiet?  I never flew a P51 for real so Idk.

Does anyone know a good place that explains p51 engine behavior in good detail?  Chucks guide doesn't seem to cover this exactly. You very much can't get maximum manifold pressure in every situation up high 

Also is automatic supercharger generally the way to go?

[Theodore42]

On 5/8/2021 at 7:24 AM, -0303- said:

Been wondering about this behavior. Like, should you keep advancing throttle to keep power up until supercharger kicks in while climbing? Ok, the MP jump on supercharger kick in, that's how it worked.

 

On 5/15/2021 at 8:48 AM, sublime said:

Does anyone know a good place that explains p51 engine behavior in good detail?  Chucks guide doesn't seem to cover this exactly. You very much can't get maximum manifold pressure in every situation up high 

 

The higher you go the less dense the ambient air becomes.

MP stands for Manifold Pressure; pressure as in air density

Notice that when the engine is off the MP is 29.92 inches, or whatever the ambient air pressure is where it's parked.

The Supercharger blows air into your manifold to increase the MP above the ambient air density.

The supercharger only has 2 speeds, so to maintain the MP you have to manually open the throttle as you climb out.

The Mustang has an MP regulator that automatically keeps your MP "constant" (not really)

 

Ok, so this is what happens as you climb out:

 

Sea level at Full Throttle (FT) the MP is 61 and the MP regulator is limiting you from pressing it higher (unless you use WEP)

As you climb, the ambient air pressure goes down and the MP regulator opens the throttle to keep the MP at 61 inches.

CRITICAL ALTITUDE: FT but the MP regulator can't open up any more and you start losing MP. This is probably around 8,000ft. Above this altitude you are no longer producing max power... UNTIL the high blower kicks in!

Specific Altitude (I think is what this is called): When you are FT at "max continuous" (46 MP & 2700 RPM). This might be around 10,000ft. NOTE: this is a different altitude than 46 MP & 3000 RPM

Hi Blower: At around 17000 ft the high blower kicks in. So go back to the beginning and repeat:

Climbing at FT and 61 MP: The MP regulator will again be able to keep your MP at 61 inches for several thousand feet, but eventually --

Critical Altitude: again, the MP regulator can't open up any more and you're going to start losing MP.

Specific Altitude: MP 46 at 2700 RPM while FT

40,000ft is the ceiling for the Mustang and the MP is about 30 inches, so at 40,000ft the high blower makes the air pressure in the carburetor the same as it is on the ground, which is where we started 

 

The DCS manual for the Mustang is pretty complete imo, but you can find several authentic Mustang manuals - the engine in DCS is the V-1650-7 model

[sublime]

The mustangs critical alt then is around 14k where I get lowest performance and 15k something the blower kicks in..

As far as the propeller.  When people discuss prop pitch are they discussing the RPMs of the prop??  Does higher RPMs always equal more power or are there times where having lower RPMs with high throttle produces more speed?

[Theodore42]

Critical alt is when you are at 61 MP and 3000 RPM but going any higher would make you MP go down to 60.

 

Ya prop pitch refers to RPMs. The P51 automatically governs the prop pitch though, so max RPMs in a Mustang is always going to give you the most power.

In other aircraft without a governer you might have to decrease the prop pitch at high speed to keep the RPMs at best performance.

 

Too high MP vs too low RPMs will damage the engine, so be careful! The manual has a bunch of safe engine settings but all you need are FT / 61MP and max continuous.

[grafspee]

This is a power chart for V-1650-7 without RAM AIR, with corresponding MP values for High and Low blower.

When in climb MP starts to drop down this mean that max throttle altitude was reach, but this does not mean that performance of the plane will be restored after High blower engage.

Automatic system changes supercharger speed at optimal altitude, exact alt change over will change depend on RAM AIR.(according to manual cut in happens between 14.5k ft and 19.5k ft)

As you can see from chart, forcing supercharger to change speed below 14k has no sense, going in to high blower at 11k ft is simply lost good 150hp.

This is why 2 speed supercharger was installed in this engine 1st reason is to improve high alt power output, and 2nd reason is to keep excellent performance at low.

alt.

Boost regulator is linked with throttle,V-1650-3/-7 had less advanced system which regulation range starts at 41' in high speed range, in v-1650-9 boost regulator was maintaining MP at much wider range starting from above 20'.

It is obvious that DCS P-51 has this early boost regulator.

For me only one thing explains why we need to open throttle more in climb to maintain 46" for example.

For me this looks like that. That boost regulator is using carb's throttle valve to maintain set boost, but it is limited how much it can open close throttle valve  regarding to in cockpit throttle position.

So we are flying at SL 2700rpm 46" throttle is set and boost regulator sits in middle point of its regulation range, so we start to climb and boost regulator will open throttle more to maintain 46" but at some point boost regulator will hit regulation range for given throttle position, so boost regulator opened throttle as much as could but throttle position in cockpit is set to 60% which is limiting boost regulator at this point pilot need to add throttle.

In case that boost regulator has no such limits, so it has full authority over throttle position, then no throttle adjustment would be needed in climb.

After take off you would set 46" by throttle, with no need to touching throttle ever. Above critical alt MP would drop since throttle is wide open, as soon high supercharge speed cut in, boot regulator would instantly adjust to maintain 46" again.

 

 

 

Description says that boost regulator is designed to maintain within desirable limits, a selected manifold pressure. But unfortunately not specifics.

One we know for sure that boost regulator has some limits.

I'm not sure, does "desirable limits" refer to this 41"-61 range or to something else.

Again all manual or even engine manual, describe that automatic boost control will maintain set MP regardless of the altitude until supercharger critical altitudes.

Edited May 31, 2021 by grafspee